Telephone handsets usually are fitted with omnidirectional microphones, which offer little discrimination against background noise. As a consequence, noise may be transmitted together with the speaker's voice, and interfere with the far end party's ability to understand what is being said.
Noise cancelling microphones have been proposed as a possible solution to this problem. These microphones, sometimes referred to as pressure gradient or first-order differential (FOD) microphones, have a vibratable diaphragm which is acted upon by the difference in sound pressure between the front and back sides of the diaphragm. An electrical signal is thus produced which is proportional to the gradient in the sound pressure field at the microphone. At the telephone mouthpiece, the acoustic field due to ambient noise will generally have a smaller pressure gradient than the acoustic field due to the speaker's voice. As a consequence, the voice will be preferentially sensed and transmitted relative to the ambient noise.
U.S. Pat. Nos. 4,584,702, 4,773,091 and 4,850,016 describe designs for incorporating a pressure gradient microphone into a telephone handset. Although useful, these designs fail to take into account all of the acoustic spatial information that might be used to enhance the speaker's voice relative to ambient noise. Moreover, the frequency response of pressure gradient microphones generally causes a change in the frequency content of the transmitted voice that becomes more noticeable as the distance from the speaker's mouth is increased. This tendency is readily offset by electronic frequency shaping. However, the use of electronic frequency shaping tends to partially counteract the ability of the microphone to reject noise. Thus, if frequency shaping is to be used, it is desirable to have a microphone with improved noise-rejection characteristics so that some marginal loss of performance can be tolerated.
In order to achieve still better noise rejection, practitioners in the microphonic art have proposed the use of second order differential (SOD) microphones, which measure a spatial second derivative of the acoustic pressure field. A ratio can be taken of two such second derivatives, the numerator corresponding to the speaker's voice (near the lips), and the denominator corresponding to the ambient noise field. Generally, this ratio will be significantly greater than the analogous ratio of first derivatives (such as would characterize the performance of a FOD microphone). Consequently, a SOD microphone is expected to exhibit much greater sensitivity to a speaker's voice relative to ambient noise than a FOD microphone.
SOD microphone designs have been described, for example, in A. J. Brouns, "Second-Order Gradient Noise-Cancelling Microphone," IEEE International Conference on Acoustics, Speech, and Signal Processing CH1610-5/81 (May 1981) 786-789, and in W. A. Beaverson and A. M. Wiggins, "A Second-Order Gradient Noise Canceling Microphone Using a Single Diaphragm," J. Acoust. Soc. Am. 22 (1950) 592-601. In general, these designs are configured to measure a second order derivative of the acoustic field near the speaker's lips, but they do not optimally exploit the spherical wave nature of the speaker's voice field to maximize sensitivity to the speaker's voice. As a consequence, the voice response of prior art SOD microphones is very sensitive to the distance R from the speaker's lips. Specifically, the voice response is expressible as the sum of three terms: a frequency-independent term inversely proportional to R.sup.3, a term proportional to the angular frequency co and inversely proportional to R.sup.2, and a term proportional to .omega..sup.2 and inversely proportional to R. That is, with increasing distance from the lips, prior art SOD microphones very soon exhibit an undesirable, .omega..sup.2 component of the near-field voice response. This effect tends to reduce the net transmitted voice power, and to make the voice sound deficient in low frequencies.